Background: The AHCPR funded Cost-Utility of Stroke Prevention (CUSP) Project collaborates with the NIH-funded Stroke Prevention in Atrial Fibrillation project in evaluating the effectiveness of aspirin and warfarin in the prophylaxis of stroke in patients with nonvalvular atrial fibrillation (NVAF). NVAF is a major risk factor for stroke in patients with embolism. The projects were originally intended to end in December 1991, but there has been a major change in the protocol necessitating an extension of funding. Both aspirin and warfarin appear significantly better than placebo, so much so that the placebo arms were dropped from the studies in January 1990. Objectives/Aims: The CUSP project is designed to evaluate the detailed health related quality of life (HRQOL) effect of aspirin and warfarin prophylaxis in subjects with NVAF. Specific questions include, "Is the net HRQOL for one therapy greater than the other?" and "Since it appears aspirin loses some of its effectiveness in patients over the age of 75, is the HRQOL of the therapies different in this geriatric population?" Design: Multicenter, randomized trial of aspirin or warfarin. Subjects are enrolled into CUSP by SPAF personnel Quality of Well-being (QWB) interviews are performed over the telephone by CUSP staff at entry, annually thereafter, and after a SPAF event or other significant change in health status.